Focusing device with permanent magnets for electron tube and electron tube equipped with such a device

Abstract

A focusing device for an electron tube is constituted by alternating magnets and pole pieces in the form of washers, which are fixed against one another in such a way as to form a cylinder surrounding the envelope of the tube and being of the same axis as the latter, the electron beam propagating along said axis. The magnetized washers have a central hole and are interrupted over part of their circumference by an opening permitting their fitting between the pole pieces previously fixed to the envelope.

What is claimed is: 
     
       1. A focusing device for an electron tube comprising alternating permanent magnets and pole pieces in the form of flat washers having a central hole and fixed against one another around the envelope of the tube in the electron beam propagation direction, wherein the permanent magnetic washers are open over part of their circumference, the spacing between the edges of the opening being sufficient to permit the passage of the envelope. 
     
     
       2. A focusing device according to claim 1, wherein the edges of the opening consist of two straight sections of the washer, which are parallel to one another and separated by a distance less then than the diameter of the circular central hole. 
     
     
       3. An electron tube and in particular a travelling wave tube, incorporating a focusing device according to either one of claims 1 or 2.